The Sun's disk formed Earth about 4.5 billion years ago. It was molten in the beginning. A solid crust formed after it cooled. Life became a possibility when the atmosphere cooled.
How did that happen? The atmosphere was rich in carbon and it had to be removed before the temperature could drop and Earth could become a place to live.
Where did all the carbon go?
The Hadean eon is the name for the first 500 million years of Earth's existence. The Greek God of the Underworld is referred to as the name. It is an informal name for Hell.
The Hadean eon is named after it. Earth was still hot even after it began to cool. 100,000 times the current level of atmospheric carbon was contained in the atmosphere. Venus has a thick atmosphere that traps heat and keeps temperatures high. During the Hadean, the Earth's surface temperature would have exceeded 200 Celsius.
Earth had to scrub a lot of carbon from its atmosphere before it could cool. It is difficult to piece together events on the very young Earth. The geological evidence is not very strong.
A pair of researchers think they have a new explanation for removing atmospheric carbon, and it involves a type of rock that no longer exists.
A new research article titled "A wet heterogeneous mantle creates a habitable world in the Hadean" presents the team's findings. The first author is a PhD student at Caltech. Jun Korenaga is a professor of Earth and planetary sciences at Yale. The study was published in Nature.
The most complete theory for Earth is presented in the press release.
The Hadean was both enigmatic and dynamic. The planet went through a lot of changes over 600 million years. Earth's surface environment was similar to present-day Earth by the middle of the Hadean.
There were a few things that had to happen. There was a lot of atmospheric carbon that had to be removed.
It was a magma ocean, a sphere of molten rock and nothing else. The planet was forming during this phase. A large object is created in the accretion process. It took between 70 million and 100 million years for the Earth to assemble. Planetesimals slammed into the Earth-to-be, generating heat and keeping it molten.
The life of rocky planets depends on the magma ocean. The liquid state allows heavier elements to sink to the core and lighter elements to float on top. This is how planets are differentiated into a core, mantle, and a crust. Without the outer core, Earth would not have a protective magnetosphere and probably no life.
Massive quantities of greenhouse gases were released when the Earth's magma ocean solidified. Earth's early atmosphere contained a lot of CO2 and H2O. The young planet has an extreme climate. Things had to change rapidly for that climate to become more moderate. The rock was the only place where the greenhouse gases could be kept. Carbon is sequestered into rock through the transformation into carbonate minerals in ocean basins. The carbonates are part of the mantle. Some of the carbon we are emitting will be subducted and end up as diamonds in the distant future.
The question is about the amount of time involved. The carbon sequestration had to be very efficient if the climate was moderate and similar to modern Earth. How did it all work?
The kind of prehistoric rock that made Earth habitable was said to be by Korenaga and Miyazaki. They are called high-magnesium pyroxenites. There are no rocks on Earth today, according to the researchers. The rapid carbon removal model they developed shows that the rocks must have existed. They know what they would look like.
The rocks were enriched in a mineral called pyroxene and had a dark greenish colour.
Carbon dioxide has an affinity with magnesium minerals. carbonates are sequestered into Earth's mantle. If there were enough high-magnesium pyroxenites, they could help account for the rapid removal of carbon from Earth's atmosphere.
The geologically rapid transformation of Earth's atmosphere during the Hadean is only one part of the explanation. Something else needed to happen because magnesium-rich minerals were plentiful. The cooling of the ocean caused the build-up of carbon in the atmosphere. Huge quantities of GHGs were released as it cooled and solidified.
Earth had a wet mantle during the Hadean eon. The layer of rock is 3,000 km thick. A wet mantle has a high proportion of water and it affects the weather.
The Hadean produced molten silicate minerals in the Earth's mantle. Water lowers the melting point of silicates. The mantle experienced convection because of the currents in the molten material. More of the magnesium-rich minerals were brought to the surface where they could react with carbon. The surface mantle recycled itself more rapidly, bringing magnesium into contact with carbon. The carbon was removed from the atmosphere and put into the mantle.
The mantle had to be different. The Hadean evolution is explained by the mantle with a chemical.
The authors propose that rapid recycling is possible with a heterogeneous mantle.
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